A review of the state of cloning science in the journal Science by an American expert concludes that "our understanding is stalled at a fundamental level". Jose Cibelli, of Michigan State University, supports therapeutic cloning, but points out that the basic science is still not well understood. Scientists, he says, have focused too much on "goal-directed" experiments and not on understanding how cloning actually works. They still do not know, for instance, which genes are responsible for reprogramming a cell when it is cloned.

"Unveiling the genes and pathways involved in the cloning procedure is the first step to creating reasonable approaches for generating human cells that can later be used in therapy," he writes. "Only then will so-called (and still hypothetical) therapeutic cloning become obsolete."